Water injected compressors are used for compressing gaseous media such as air in order to make them available as a gas. Water is used for sealing, lubrication and cooling of the compressor. The injected cooling water is separated from the compressed gas after exiting the compressor. The cooling water heated by the compressor is fed to a cooling device. The cooled cooling water is then filtered and fed back into the compressor. If the cooling water reaches a high conductivity as a result of the heating (above 50° C.), then calcification significantly increases, thus causing dirt/debris to occur that impairs functioning (jamming of valves, altering spaces).
In such compressors, there may be variation of several parameters of the cooling water due to evaporation of the cooling water and/or absorption of atmospheric humidity into the cooling water. Evaporation of cooling water causes the relative mineralization of the cooling water to increase, thereby increasing sediment that can lead to compressor damage. In the opposite case, if the mineral content of the cooling water decreases due to atmospheric humidity absorption, then this has a negative effect on the buffer capacity of the cooling water to absorb free carbonic acid. Free carbonic acid in the cooling water that is not bound, is very aggressive and promotes corrosion. The pH of the cooling water may be altered by evaporation of cooling water, absorption of atmospheric humidity into the cooling water, or absorption of copper or iron ions, so that the cooling water has a corrosive effect.
DE 699 11 695 T2 discloses a screw-type compressor, in which the compressed air is cooled to a saturation temperature, the water content is condensed and separated, and the separated water content is supplied to the compressor, while excess circulating water is discharged from the water container. The recovered water contains only few impurities and balances the demineralized water. The amount of condensed water is usually greater than the amount of water which is lost through evaporation, so that continuous operation for many hours is possible without replenishment of water. The tap water used for the initial filling of the water tank that includes some impurities may be converted into pure (demineralized) water within a short time by the method described without the use of a demineralization plant. The problem here is that demineralized water cannot bind dissolved carbonic acid and thus the pH decreases. A reduced pH leads to increased corrosion.
EP 0 800 622 B1 discloses a compressor installation with a water-injected compressor, in which the cooling water that is free of additives is circulated in a circulation circuit. A water treatment device binding mineral deposits is arranged in the circulation circuit. The injection into the compressor is so designed that the compressor performs nearly isothermal compression, whereby the temperature of the outlet gas overshoots or undershoots the inlet gas temperature by less than 10 K. The compressor system described above may be considered as a quasi-closed system that allows operation with a closed circuit. If the gas to be compressed is supplied with a quantity of moisture that also leaves the compressor installation in the compressed air, the quantity in the cooling water remains constant and the circulation circuit may be operated closed to the outside.
US 2009/0232688 A1 discloses a compressor system comprising a water-injected compressor with an inlet pipe and an outlet pipe, an air-intake filter connected to the inlet pipe, a water separator into which the outlet pipe empties and a recirculation pipe arranged between the water separator and the compressor. The air-intake filter comprises a housing having an air inlet and an air outlet, to which the inlet pipe is connected, and a substrate disposed in the housing in the form of a material with an open cell structure through which the air is sucked in. The air-intake filter comprises a wet filter to which a water supply is connected. The water supply comprises at least a pipe connected to the water separator and a supply device for additional water. Upon falling below a predetermined level in the water, additional water is supplied. The additionally supplied water is chemically pure, i.e. it does not affect the mineralization or the pH of the cooling water.
DE 197 29 498 A1 discloses a compressor system having a compressor with water injection cooling, and in the cooling water circuit of which is arranged a measuring device for determining the conductivity of the cooling water. The compressor unit further comprises a water supply source for the supply of non-demineralized water and demineralized water as well as a regulating device. The regulating device controls the supply into the cooling water circuit from the water supply source of demineralized water in the case of overshooting an upper conductivity limit and the supply of non-demineralized water in the case of undershooting a lower conductivity limit. In this way, the pH and the mineralization of the water should be so controlled that corrosion by the cooling water and sedimentation in the cooling water is reduced. A problem with this solution is that the ion concentration of other ions, i.e. the number of copper ions in the cooling water circuit increases over time through frequently unavoidable corrosion processes in the cooling water circuit, i.e. by corrosion of copper. These other ions lead to an increase in conductivity, although the amount of calcium carbonate does not change. Due to the increased conductivity of the cooling water, the addition of demineralized water now decreases the calcium carbonate content in the cooling water below a desired minimum value. This means that the carbon dioxide dissolved during the compression is no longer bound by the hardness minerals (buffer effect of the calcium carbonate), and thus the pH decreases. However, a reduced pH will then lead to an increased corrosion of the parts of the compressor.
DE 101 51 176 B4 discloses a compressor system with a water-injected screw compressor for compressing gas. The screw compressor is connected to a cooling water circuit. The cooling water is injected either from a fresh water supply or a condensation plant depending on the cooling water circuit. A conductivity measurement and evaluation device for monitoring the concentration of ions is connected to the cooling water circuit, which in turn is connected to a control and regulating device. An independently formed condensation plant is arranged in the area of the suction passage of the cooling water circuit to supply a certain quantity of condensate to the screw compressor via the suction passage, so that the ion concentration in the cooling water circuit remains constant within predetermined tolerance limits. In this solution, conductivity measurement also takes place in the cooling water circuit, but this is also associated with the disadvantages described above.
DE 101 51 175 A1 describes a compressor and a method which involves subjecting water injected into or before injection into the compressor to a magnetic field intensity of a magnet arrangement determined by a parameter in order to increase the lubrication capability of the water. The method regulates the conductivity of the cooling water inside the water circuit of the compressor.
DE 103 07 803 B4 describes a compressor system including a compressor with water injection cooling supplied from a cooling water circuit, and a condensing unit for the condensing of water present in the compressed medium. A device is provided to measure the conductibility of the condensate. A control unit is provided to regulate the conductibility of the condensate.
DE 10 2004 053 895 A1 contains a compressor and a method for the exchange of water in a compressor with water injection. The exchange of at least part of the water takes place in a program-controlled manner in dependence upon parameters and/or signals from sensors. A conductivity sensor can be arranged in the water circuit.